1. Field of the Invention
The present invention relates to an image forming apparatus for visualizing an electrostatic latent image formed in such a process as an electrophotographic process, an electrostatic printing process and an electrostatic recording process.
2. Description of the Related Art
According to the colorization of an image forming apparatus in recent years, a non-magnetic one-component color toner using no magnetic carrier is often used in general-purpose color printers, and a magnetic brush developing method using a two-component developer is generally employed in high-speed printers.
In the electrophotographic process, a photoconductive photoreceptor is charged and exposed to form an electrostatic latent image on the photoreceptor. The electrostatic latent image is then developed with a toner in a fine particle form including a colorant and a resin as a binder. The resulting toner image is transferred and fixed to recording paper to obtain a recorded image.
In a latest color image forming apparatus, an intermediate transfer material, such as an intermediate transfer belt, is used, in which the toner is transferred from the photoconductive photoreceptor to the intermediate transfer material to form plural toner images on the intermediate transfer material, and the toner images are then transferred to recording paper.
It is an important step in an image forming apparatus that a toner image developed on a photoconductive photoreceptor is transferred to recording paper to obtain a final image.
There are some cases in the image forming apparatus that dropout occurs upon transferring, whereby white spots and density unevenness are formed in a final image on a transfer material, such as recording paper, or the image is locally not transferred to form so-called worm holes.
The white spots and density unevenness are confirmed as unevenness in a solid image with a large area or failure in gradation in a half-tone image.
In order to avoid the white spots and density unevenness, such a proposal has been made that relates to a particle size distribution of a toner (for example, in JP-A-4-204660 and JP-A-6-175391) and such a proposal has been made that relates to a weight of a residue remaining after sieving a toner (for example, in JP-A-2000-137351).
However, the proposals are still insufficient to leave rooms for improvement although there have been tendencies to avoid the white spots and density unevenness.
There is a proposal for avoiding transfer failure like wormholes by defining the circularity of the toner (for example, in JP-A-10-097095), but there are cases where the transfer failure occurs depending on recording paper.
The toner amount that is consumed upon printing in an image forming apparatus using a color toner often varies largely depending on the kind of printed images. For example, a large amount of a toner is consumed upon printing an image with a large printing density, such as a graphic image, and a consumed amount of a toner is significantly small upon printing an image with a small printing density, such as an image including only characters. The toner consuming amount largely varies depending on an image to be printed, and in the case where many images with a small printing density are printed, the toner is agitated in the developing device in many times, whereby an external additive attached to the toner surface is buried in the toner due to the agitation stress in the developing device to cause deterioration in fluidity and change in charging property, which facilitate occurrence of transfer failure.